Small directional antenna system



1961 c. B. WATTS, JR 2,972,145

SMALL DIRECTIONAL ANTENNA SYSTEM Filed 061;. 6, 1959 U ifll /7 v INVENTOR. 12 gh zera mam:

4 TTORNEYS United States Patent SMALL DIRECTIONAL ANTENNA SYSTEM Chester B. Watts, Jr., Alexandria, Va., assignor to Scanwell Laboratories, Inc., Springfield, Va., a corporation of Virginia Filed Oct. 6, 1959, Ser. No. 844,797

8 Claims. (Cl. 343-742) This invention relates to directional antennas which are small compared to the wave length of the energy passing therethrough, and more specifically to a system for feeding a pair of spaced coplanar loops in a manner to produce a stable unidirectional radiation pattern.

While the system is described with reference to radio waves, the system can be equally well applied to sonic waves, in which case the loops are replaced with sonic transducers.

It is an object of this invention to provide, in a structure small compared with the wave length, a radiation pattern which is essentially single-lobed, and which remains so over a band of frequencies.

An embodiment includes two identical radiating elements, spaced a small distance apart, and a feeding circuit which provides, simultaneously, two symmetrical modes of current excitation to the radiating elements. In the first mode, the element currents are equal and directed so that the radiation fields at a distance tend to aid each other. In the second mode, the element currents again are equal, but are oppositely directed so that the radiation fields at a distance tend to buck each other. Considering one of the bucking mode currents as a reference, the feeding circuit provides aiding mode currents whose phase angle is substantially 90 degrees relative to the bucking mode currents, and whose amplitude is sub stantially directly proportional to frequency.

Fig. l is a sectional view of a form of device which embodies this invention;

Fig. 2 is a schematic circuit diagram of a device which embodies this invention;

Fig. 3 is a polar graph showing amplitude plots of aiding and bucking array factors; and

Fig. 4 is another polar graph showing two amplitude plots, one being of the combined array factor, and the other being of the combined array factor multiplied by the element pattern.

In the form of the invention shown in Fig. 1, numeral 3 designates a conductive housing having an input terminal portion 1 for connection to a signal supplying circuit. The housing 3 is formed to define a pair of spaced torus-shaped shielding portions 15 and 16 having air gaps 20 therein. A first loop antenna element 9, formed of solid wire or rod, is housed in shielding portion 15 and is electrically connected, at one of its ends, to housing 3 at a junction 14. The other end of element 9 is electrically connected, at 22, to one end of innerconductor 4 of a coaxial transmission line 6. The line 6 comprises the inner conductor 4 and an outer conductor 24 separated by a layer of insulation 26. The line 6 is conformed to define a second loop antenna element 10 in shielding portion 16. The other end of inner conductor 4 is electrically connected to a conductor at junction 2. The outer conductor 24 is electrically connected, at one end, to the housing 3, at junction 13.

An impedance device 11 is connected to element 9 adjacent junction 22 and an impedance device 12 is connected to outer conductor 24 at the end thereof adjacent Patented Feb. 14, 1961 ice junction 22 and forms therewith a junction 8. Both impedance devices 11 and 12 have their other ends connected to conductor 5 at junction 17.

The impedance devices 11 and 12, while shown as fixed resistors, may in fact be any type of impedance elements but are constructed or adjusted so that their impedance values are equal at the desired frequency. The particular impedance value of each of the devices 11 and 12 determines the frequency range within which the desired results are produced, as will be described, and those values may obviously be made adjustable, in any wellknown manner.

While it is to be understood that the system can be used either for transmitting or receiving, the explanation here is from the transmitting viewpoint. Radio frequency power applied to input terminal 1 results in an R.F. voltage V between junction 2 and housing 3. Current flows from junction 2 into conductors 4 and 5. Conductor 4 is associated with the bucking mode, while conductor 5 is associated with the aiding mode.

Considering first the bucking mode, R.F. power carried between conductor 4 and housing 3 enters the inner con-' ductor 4 of coaxial transmission line 6, emerging to produce a voltage V between junctions 7 and 8. If the length of transmission line 6 is small in comparison with the wave length, voltage V is substantially equal to voltage V The voltage V causes currents to flow in the symmetrical structure which comprises loops 9 and 10, and equal impedances 11 and 12. Loop 10 is the the housing at point 13. Loop 9 does not contain an inner Y conductor and may therefore be fabricated with solid rod.

It is otherwise similar to loop 10, and is connected to the housing 3 at point 14. Loops 9 and 10 are enclosed in electrostatic shields 15 and 16, respectively. By virtue of the symmetry of the structure, the total impedance between junction 7 and the housing is the same as the total impedance between junction 8 and the housing; the currents and voltage drops which exist in the various parts of one half of the structure are all respectively equal and opposite to the currents and voltage drops which exist in the corresponding parts of the other half. This means that junction 17, between equal impedances 11 and 12, must be at zero potential with respect to the housing. Insofar as the bucking mode is concerned, junction 17 could just as well be grounded to the housing; no component of bucking mode current flows in conductor 5.

Considering nextthe aiding mode, R.F. power carried between conductor 5 and housing 3 produces a voltage at junction 17 with respect to the housing which is the same as V if conductor 5 is short. Currents flow out of junction 17 into equal impedances 11 and 12. Again making use of the symmetry, the total impedance in each half of the structure is the same. The currents flowing in equal impedances 11 and 12 are therefore equal to each other as are also the voltage drops across them. Thus junctions 7 and 8 each have the same potential with respect to the housing. The equal currents which flow in loops 9 and 10 are polarized in the same direction. Since there is no potential difference between junctions 7 and 8, these points could just as well be joined by a conductor insofar as the aiding mode is concerned; thus no component of aiding mode power enters transmission line 6; no component of aiding mode current flows in conductor 4.

The two modes, bucking and aiding, can exist simultaneously in the structure of Fig. 1 independently and without coupling to each other except at the common junction 2. Here they are each excited with currents which have values inversely proportional to the respective impedances presented to junction 2 through conductors 4 and 5.

Fig. 2 is a schematic of an electric circuit which is essentimly equivalent to the structure shown in Fig. 1. With a single exception, the foregoing description given for the structure in Fig. 1 can be read with respect to the circuit in Fig. 2; the exception is that the coaxial transmission line 6 in Fig. l is replaced by an ideal oneto-one transformer 18 in Fig. 2. The inner conductor 4 and outer conductor of transmission line 6 can, however, be considered as a one-to-one transformer itself.

It should be recognized that an alternate physical structure can be designed which is based on the circuit of Fig. 2 and which employs transformer 18 as shown. In this case, loop 10, as well as loop 9, would be fabricated with solid rod. In Fig. 2, reference numerals the same as those of Fig. 1 are used to indicate corresponding junctions and/or elements.

In order to calculate the radiation patterns of-the antenna structure, it is desirable to know the loop currents for each mode in terms of the common junction voltage V If the loops 9 and 10 are small in comparison with the wave length, their impedance is almost purely inductive. For the purpose here of calculating loop currents their resistive component will be therefore neglected. It is assumed, for the sake of further simplification, that the equal impedances 11 and 12 each have the purely resistive value R, which is made large in comparison with the loop reactances. If L is the total inductance of the two loops 9 and 10 in series, then the bucking mode loop currents 1,, are given to a first approximation by V Ali i in which In the aiding mode, the loop currents I are given to a first approximation by Considering next the radiation fields, it is well known 4 that two elements carrying currents that are equal but opposite have an array factor F given by F =j2l sin sin i 3 whereas two elements carrying currents that are equal and in the same direction have an array factor F given by F,=2I, cos sin a) 4 in which:

s=element separation A=wave length =azimuth coordinate measured from a normal to the line joining the elements.

Applying the restriction s the angle is'never larger than a small fraction of a radian, so that the sine of the angle is essentially equal to the angle, while the cosine does not differ much from unity. Thus Equations 3 and 4 can be written as more simple approximations:

The shapes of these array factors are as shown plotted in Fig. 3, curves 30 and 32, respectively.

From Equations 5 and 6 it will be noted that if the currents l and I were to be equal, then the array factors would be in quadrature phase relation, while their magnitude ratio would be inversely proportional to wave length. However, in the feed system described, l and 1,, are not equal to each other, but have the values given by Equations 1 and 2, respectively. Substituting these values in Equations 5 and 6 yields:

sir. 7)

where c= ;:;=vel0city of light (constant) F..= v.% a (8) Note that the dependence on frequency has been eliminated, and the array factors are in phase. Now in order to have the array factors combine to produce optimum directivlty, it is necessary that the amplitudes be equal, that is:

2 vs vii This defines a required value, R, for the equal impedances 11 and 12:

Under the foregoing conditions, the bucking and aiding array factors superimpose to form a total array factor F which is the cardioid shape plotted in Fig. 4 as curve 34.

To obtain the resultant pattern of the complete antenna system, one multiplies the total array factor by the individual loop characteristic F The latter is a figureof-eight:

F =K sin 5 (11) Thus, the resultant pattern F of the antenna has the shape given by and shown plotted in Fig. 4 as curve 36. In Equations l0, l1 and 12, K K and K are arbitrary constants. Within the limitations of the stated assumptions this pattern shape remains stable and independent of frequency.

While a single specific embodiment of the invention has been shown and described in detail, it is to be understood that other forms may be resorted to within the scope of the appended claims.

I claim:

l. A directional antenna system comprising; two identical antenna elements spaced apart less than onetenth wave length, a feeding circuit for simultaneously feeding bucking and aiding modes of current to said elements, said aiding mode currents having a phase angle of substantially degrees relative to said bucking mode currents and an amplitude relative to that of said bucking mode currents substantially proportional to frequency.

2. A directional antenna system comprising; two identical elements and a circuit for simultaneously'feeding said elements bucking and aiding modes of symmetrical current excitation, said aiding mode currents having a bucking mode currents substantially proportional to frequency.

3. A directional antenna system comprising, two spaced radiating elements fed from opposite ends of the secondary winding of an isolation transformer, the primary winding of said isolation transformer being connected between ground an an input terminal for said elements, a pair of equal impedance devices connected between said input terminal and each of said opposite ends of said secondary winding.

4. A directional antenna system comprising; two similar spaced radiating elements, an isolation transformer, two equal impedance devices and an input terminal; said radiating elements being connected to opposite ends of the secondary of said isolation transformer, the primary winding of said isolation transformer being connected between said input terminal and ground, one of said equal impedance devices being connected between said input terminal and one end of said secondary winding, the other of said equal impedance devices being connected between said input terminal and other end of said secondary winding.

5. A directional antenna system comprising; two spaced radiating elements and a circuit for simultaneously deeding said elements bucking and aiding modes of symmetrical current components, said aiding mode components having a phase angle of substantially 90 degrees relative to said bucking mode currents and an amplitude relative to said bucking mode components substantially proportional to frequency, said aiding mode component amplitude being adjusted to produce a radiation pattern null in the direction of an extension of the line joining said radiating elements.

6. A unidirectional antenna comprising; two electrostatically shielded coplanar loops, one of said loops being wound of solid wire, the other of said loops being wound of coaxial transmission line, one end of each loop being grounded, the other end of each loop being connected through each of two equal impedance devices to a common input terminal, the inner conductor of said coaxial transmission line being connected at one end to said common input terminal and at the other end to the ungrounded end of said first loop.

7. A unidirectional antenna as defined in claim 6, each of said equal impedance devices having a resistance substantially equal to twice the inductance of the two loops, connected in series bucking, multiplied by the ratio of the velocity of light to the loop separation.

8. A unidirectional antenna comprising; two coplanar loops, an input terminal, balancing transformer means connecting said input terminal to said loops in bucking polarity, two equal impedance means connecting each of said loops to said input terminal in aiding polarity, each of said equal impedance means having a resistance substantially equal to twice the inductance of the two said loops, connected in series bucking polarity, multiplied by the ratio of the velocity of light to the separation distance of the two said loops.

No references cited. 

